In a general manner, in the field of the automated transfer of workpieces from one station to another station, various systems have been developed and are currently used.
In some of these systems, self-guided autonomous carriages comprise an electric motor powered by a battery of accumulators which the carriages carry. The battery is rechargeable with energy at the work stations or in parking zones.
These motorized carriages are of large dimensions approximately 0.5.times.1 m to 1.times.2 m) and are generally used for transporting voluminous and/or heavy workpieces (from 25 kg to 500 kg) and circulate at low speed (10 m/min to 40 m/min on average).
These pieces of automatic handling equipment move inside production workshops along routes which have been partially or totally defined previously. They have piloting means integral with the carriages so as to slave their steering and their circulation speed as a function of a permanent or discontinuous detection of the route traveled and of that still remaining to be traveled.
This technique envisages numerous embodiments which allow these carriages to circulate in complete safety by means of on-board sensors of various natures. The types of sensors include electromechanical sensors, especially combined with bumpers actuating one or a plurality of operating switches. Inductive, optical, telemetric, among other sensors may also be used. These environment sensors are combined with the circuits for slaving the clutches, brakes, and speed variators of the motor or motors of these carriages. They control the braking, deceleration and stopping so as to avoid any abrupt collision with other carriages or obstacles or else their slow advancement and their stopping in each positioning zone provided at the work stations. The work stations often comprise the means for recharging carriages' batteries.
Moreover, the progress made in miniaturizing motors and sensors, combined with widespread, inexpensive control electronics, have permitted this well-known technology to be transferred to applications relating to the entrainment of pallets of small dimensions (0.3 m.times.3.4 m) in installations provided for machining and/or assembling workpieces of low weight (from 1 kg to 45 kg) circulating at high speed (120 m/min).
Numerous embodiments calling upon motorized pallets exist. These pallets are of a size very close to that of remotely guided toy cars and their respective motorization method is generally similar to that of such toys. They are equipped with an electric geared motor supplied from a battery which they carry. The motor is slaved in speed by various electronic circuits managing the acceleration and braking of two or four drive wheels rotating about horizontal axes.
Taking this prior art into account, there have more recently been proposed embodiments relating to installations for assembling and/or machining for the manufacture of workpieces of small dimensions carried by motorized pallets. Embodiments can be distinguished from one another generally by the constitution of the entrainment means of these pallets interacting with straight or curved devices for guiding them and suitable for ensuring their changes of direction on a floor, on rails, or on other supports.
The most recent installations in this field are based on the "off line" working principle. The majority of assembly and/or machining operations take place there, outside of a general circulation line, in circuits which bypass this line. The manual and/or automatic work stations are, therefore, outside of the general high-speed circulation line of the pallets, in a bypass circuit, along which the assembly and/or machining operations are carried out.
The most compact installation is obtained when the changes of direction of the pallets are performed at a right angle without changing the spatial orientation of the pallets.
But, on the other hand, to make these installations able to be exploited as universally as possible, often implies that during a change in direction, these pallets modify their orientation. For example, in the case in which reasons of ergonomics or the technical necessities are more important than the criteria of the bulk of their bend surface area.
The latest progress achieved in the architecture for mounting the various pieces of base equipment led to the production of more and more compact installations comprising a very high density of circulation tracks with numerous crossroads and changes of direction which are very often less than 0.5 m apart.
Under these conditions, the top speeds of more than 100 m/min indicated by the motorized pallet constructors are not very significant, because they are almost never reached in very compact installations.
The true overall exploitation performances of these installations are, in fact, essentially linked to the acceleration and deceleration performances of the pallets, mainly in the bends which occur at very short intervals. Moreover, the braking capability of the pallets without skidding is also a determining factor when they circulate at high speed. The risks of collision are not excluded, especially when one pallet changes circulation direction leaving the main circulation track, without changing its spatial orientation, so as to reach an "off line" station situated on a track in bypass. This pallet therefore constitutes an obstacle for the pallet which follows.
A related problem resides in the difficulty of making the pallets circulate at high speed without the workpieces. The workpieces are often unstable in the phase in which they are placed on the pallet, before being assembled or machined. As a result of this instability, the workpieces may be thrown off due to the vibrations and shocks generated in the very short bends of the pallets.
Finally, in a general manner, it is desired to cause the pallets to circulate for many hours, without cleaning the installation. But, it has been observed that, in such a case, the adherence of the self-driven pallets on a floor or the like becomes random due to soiling in the form of grease, oil, and/or shavings, among other causes, which affects the circulation line. The effect of the soiling is more harmful the greater the weight of the load transported.